The present invention relates to a double differential summing amplifier with four independent inputs.
In the electronic processing of analog signals, such as are encountered in operational amplifiers, comparators and sample and hold circuits, it is often necessary to effect linear combinations of sums and/or differences of given signals, optionally following amplification. The state of the art provides no apparatus meeting this requirement and the first objective of the circuit according to the invention is to meet this requirement.
Such a circuit has four amplifying and/or summing branches to which are applied four input voltages and the circuit supplies two output voltages, the difference of which constitutes the amplified voltage. A second objective of this circuit is to solve the problem of the known abnormalities of the transfer curve of a differential amplifier and its operating point, either by means of an external control or automatically.
It is well known that when use is made of MOS transistors in integrated circuits, it is not possible to carry out a precise check or control with respect to regulation of the operating points of each of the components considered in isolation.
In general terms, the response curve of a differential amplifier has the standard configuration shown in FIG. 1. On the ordinate is plotted the output voltage Vs as a function of the input voltage Ve on the abscissa. Conventionally, the amplification curve 1 does not pass through the origin of the coordinates, which means that for a zero input voltage, the output voltage is not equal to zero. This known shortcoming of differential amplifiers is obviated by providing an offset voltage, which is equal to segment OM in FIG. 1, which represents the voltage which has to be applied to the input of the amplifier to collect a zero voltage at the output.
In the prior art amplifiers, the operating points have hitherto been corrected by several more or less effective and precise methods, such as the adjustment of one of the input voltages with the aid of an external potentiometer or by the control of a resistor with the aid of a laser or, as in the case of the amplifier circuit forming the object of French patent No. 2 318 534 in the name of the present Applicant with the aid of a common mode feedback system, or through the use of a half-branch of the amplifier, etc. To these different corrections are added dynamic problems during operation.